Capping of metal interconnects in integrated circuit electronic devices

ABSTRACT

A multilayer metal cap over a metal-filled interconnect feature in a dielectric layer for incorporation into a multilayer integrated circuit device, and a method for forming the cap.

BACKGROUND OF THE INVENTION

This invention relates to integrated circuit manufacture and, inparticular, to metal-based capping of interconnect metallization.

In damascene processing metallization is employed to form electricalinterconnects in an integrated circuit substrate by metal-filling ofinterconnect features such as vias and trenches formed in the substrate.If the metal deposited on such a substrate is Cu, it can diffuse rapidlyinto the Si substrate and dielectric films as, for example, SiO₂ or lowk dielectrics. Copper can also diffuse into a device layer built on topof a substrate in multilayer device applications. Such diffusion can bedetrimental to the device because it can cause electrical leakage insubstrates, or form an unintended electrical connection between twointerconnects resulting in an electrical short. And the correspondingdiffusion out of the interconnect feature can disrupt electrical flowtherethrough.

Metal deposited on the substrate also has a tendency to migrate out ofthe interconnect feature when electrical current passes through thefeature in service. Electron bombardment of the metal moves the metalout of the feature. This migration can damage an adjacent interconnectline, cause junction leakage, form unintended electrical connections,and disrupt electrical flow in the feature from which the metalmigrates.

Accordingly, among the challenges facing integrated circuit devicemanufacturers is to minimize diffusion and electromigration of metal outof metal-filled interconnect features. This challenge becomes more acuteas the devices further miniaturize, and as the features furtherminiaturize and densify.

Another challenge in the context of metal interconnect features is toprotect them from corrosion. Certain interconnect metals, especially Cu,are more susceptible to corrosion.

Copper is a fairly reactive metal which readily oxidizes under ambientconditions. This reactivity can undermine adhesion to dielectrics andthin films, resulting in voids and delamination. Another challenge istherefore to combat oxidation and enhance adhesion between the cap andthe Cu, and between structure layers.

To address these challenges the industry has employed a variety ofdiffusion barrier films as a cap over Cu and other metal interconnectfeatures. Refractory metals and their alloys have been deposited in thinfilms by physical vapor deposition (PVD). SiN and SiC have beendeposited for this purpose by chemical vapor deposition (CVD). Alimitation of SiN and SiC is that they have a relatively high dielectricconstant (k value), which tends to increase capacitance of theinterconnect. An increase in capacitance can increase power dissipationdue to resistance/capacitance coupling (RC delay), thereby limiting theperformance.

In general, barrier or capping layer formation by blanket vapordeposition is expensive and time-consuming, as it involves multipleprocessing steps. The deposited films need to be patterned and etched,followed by resist removal. Some degree of misalignment is expected withlithographic patterning.

Electroless Co and Ni have been discussed as a protective layer overelectrical interconnect lines in, for example, U.S. patent publicationNo. 2003/0207560.

Chemical mechanical polishing (CMP) is performed on a substrate prior tocapping and following via formation to, for example, remove unwanted Cuoverburden deposited during damascene processing and thereby planarizethe surface. This CMP can cause traces of copper to be embedded orsmeared onto the dielectric material. These traces of copper, if notremoved, can contaminate the dielectric. The traces of Cu can have adetrimental effect on the selectivity of a Co-capping process by causingdeposition of electroless Co on the dielectric between the Cu traces,which can result in junction leakage. An etchant is therefore employedin a pretreatment composition to either remove these traces of copper,undercut the dielectric on which they reside, or both.

According to conventional wet processing, sequences separate cleaningsolutions are employed for cleaning of dielectric, and for cleaning ofthe metal. Dielectric cleaner lightly etches the dielectric surface inorder to undercut metal traces embedded onto the dielectric during CMP.Metal cleaner removes surface oxides on Cu and any remaining traces ofCu embedded in the dielectric that were not removed during thedielectric cleaning step. A cleaner may be necessary to remove residuesfrom Cu inhibitors such as benzotriazole (BTA) compounds used during CMPprocessing so that such residues do not interfere with effectiveness ofactivation, uniformity of activation and initiation, smoothness of capdeposition, adhesion of capping deposit, and thermal stability of thecap.

SUMMARY OF THE INVENTION

Briefly, therefore, the invention is directed a method for forming amultilayer metal cap over a metal-filled interconnect feature in anintegrated circuit substrate for incorporation into a multilayerintegrated circuit device comprising. The method involves depositing afirst metal cap layer over the metal-filled interconnect feature in afirst deposition process which constitutes electroless metal depositionfrom an electroless bath comprising a source of metal ions and areducing agent; and depositing a second metal cap layer over the firstmetal cap layer in a second deposition process distinct from the firstdeposition process to thereby form the multilayer metal cap as apermanent component distinct from the metal-filled interconnect feature.

The invention is also directed to a multilayer cap over a metal-filledinterconnect feature in an integrated circuit substrate formed by theforegoing method, and to an integrated circuit substrate comprising amultilayer cap over a metal-filled interconnect feature formed by theforegoing method.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 is a schematic representation of a segment of an integratedcircuit device shown in cross-section.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with this invention, a multilayer metal cap is depositedover an interconnect feature in an electronic device substrate. Thesubstrate is selected from among any substrate which has metalinterconnect features, typically Cu, but in some instances Ag, in adielectric, such as SiO₂ and low k dielectrics.

In certain preferred embodiments of the invention, the cap has multiplelayers which serve distinct primary purposes, or has multiple distinctlayers which function to achieve the same primary purposes. As explainedfurther below, the two or more layers of the cap have differentchemistries, and/or are deposited by distinct methods, under distinctconditions, to distinct thicknesses, which renders two or more layersdistinct in form.

Depending on various assembly and service conditions, by having such twoor more distinct layers, the properties of the cap can be tailored to aparticular application; e.g., to be more impervious to electromigrationof interconnect metal out of the interconnect, more resistant tocorrosion, and/or more resistant to etching during etch-back ofsubsequently deposited layers of the device. It has been discovered thatbecause the respective two or more layers are distinct from each otherin terms of chemistry, microstructure, morphology, and/or otherattribute, their combined properties are enhanced in comparison to asingle continuous layer of the same overall thickness.

The purpose of the cap is to provide a distinct layer of protection overthe interconnect. Accordingly, the metal—which encompasses metal alloysin the context of this invention—is selected for the cap from amongeffective diffusion and electromigration barrier metals. Such metals arecharacterized by low diffusivity and low mobility under themanufacturing and service conditions encountered by the device. The capmetals are specifically selected to avoid diffusion into theinterconnect, because the goal is to form a distinct cap substantiallyentirely on top of the interconnect rather than an intermittent reactionlayer of intermetallics from the interconnect and cap metals. Specificmetals for the first, bottom-most layer of the cap adjacent theinterconnect are therefore selected such that they are substantiallyimmiscible with the Cu interconnect. Substantially immiscible means thatsome insignificant amount of miscibility is tolerated, but the metalsare at least sufficiently immiscible that there is no significantreduction in performance due to miscibility under expected serviceconditions. While suitable immiscibility can be determined by differentcriteria for different applications within the scope of the invention,in one aspect, the metal is selected from among metals in which Cu has asolubility of less than about 10⁴×exp((−0.75 eV/kT)at. % for atemperature between 900 and 1100° C., and less than about 10³×exp((−0.52eV/kT)at % in the temperature region between 550 and 700° C. Thesolubility of Cu in bulk Co is ˜4.5×10³×exp((−0.75 eV/kT)at. % for atemperature between 900 and 1100° C., ˜6×10²×exp((−0.52 eV/kT)at % inthe temperature region between 550 and 700° C. Among metals avoided inthe first layer of the cap are Ni, Au, and Ag due to their miscibilitywith Cu.

The cap metals in all layers are selected to avoid diffusion into theadjacent Si or other substrate material. Suitable diffusivity can bedetermined by different criteria for different applications within thescope of the invention. Among metals avoided in the second andsubsequent layers of the cap are Au and Ag due to their diffusivity andmobility in Si.

As a general proposition, Ni is avoided as a first layer on top of Cuinterconnects due to its aforementioned miscibility with Cu and becauseNi tends to alter electrical conductivity characteristics of Cu byformation of Ni—Cu intermetallics. Cobalt-based alloys, in contrast, donot significantly alter electrical conductivity characteristics of Cu.One of the several preferred combinations meeting these criteria anddescribed in more detail below involves a Co-based first layer becauseCo provides good barrier and electromigration protection for Cu.

The cap materials are selected such that they are electricallyconducting, rather than insulating. They are also selected so they canbe deposited by non-blanket deposition methods, that is, methods whichare chemically selective for Cu interconnects, or for previouslydeposited layers of the multilayer cap.

Within the foregoing material selection criteria, in one aspect theinvention is a cap having two or more continuous layers wherein at leastone layer is deposited by electroless plating, and the remaining layeror layers are deposited by either electroless plating or an immersionprocess. In another aspect of the invention one of the layers of the capis a discontinuous layer formed by an electroless process, and theremaining layer or layers are continuous and deposited by eitherelectroless plating or an immersion process. The multilayer metal cap isnot subsequently removed during processing, nor is it annealed todiffuse into the interconnect, and in this regard it is a permanentcomponent distinct from the metal-filled interconnect feature.

As shown schematically and not-to-scale in cross-section in FIG. 1, inone embodiment of the invention there is a component 10 of an eventualmultilayer integrated circuit device which includes an insulativematerial 12 constituting an inter-layer dielectric (ILD) on asemiconductor substrate 14. A typical dielectric material is BD(available from Applied Materials), or low k dielectrics available fromNovellus Systems, Inc. of San Jose, Calif. under the trade name Coral. Atypical semiconductor substrate is monocrystalline Si. An interconnectfeature at the top surface of the feature is filled with Cumetallization 16. The multilayer cap of the invention comprises firstmetal cap layer 18 and second metal cap layer 20. There is a barrierlayer such as TaN in the interconnect between the Cu metallization andthe insulative material 12, but this barrier layer is not shown in FIG.1 to preserve clarity of the schematic.

The invention is further understood by examination of at least five morespecific, nonexclusive examples from among the numerous embodimentswithin the scope of the invention. In a first example of the inventionthe multilayer cap has a first layer which comprises, e.g., Co—B,Co—B—P, Co—W—B—P, or Co—B—W deposited by borane-chemistry electrolessdeposition process employing an alkylamine borane compound such asdimethylaminoborane (DMAB), diethylaminoborane (DEAB), or morpholineborane as a reducing agent. These borane-based reducing agents render Cucatalytic to Co deposition. The process is therefore self-initiating onCu interconnects, so Co, Pd, or other seeding operation is omitted. Thisis in contrast to electroless processes based on non-borane chemistry,such as employing hypophosphite or other non-borane reducing agents,which do not render Cu catalytic to Co deposition. The non-boraneprocesses, if used to build a layer directly on a Cu interconnect,require Co seeding or other initiation mechanism. In this firstembodiment of the invention, this borane-chemistry electrolessdeposition is performed only to the extent a Co-based layer on the orderof about 5 to about 50 angstroms (e.g, about 5 to about 20 angstroms) isformed. In one version of this embodiment this first layer issubstantially entirely coalesced to provide coverage which issubstantially continuous. In a different version of this embodiment, thefirst layer is significantly uncoalesced such that its coverage issignificantly discontinuous. A second Co-based layer thicker than about100 angstroms is then deposited by an alternative deposition process,such as by electroless deposition employing a hypophosphite reducingagent, or a mixture of reducing agents. A non-borane process such as anelectroless process using a hypophosphite reducing agent is suitablebecause hypophosphite renders the Co-based first layer catalytic todeposition of a Co-based second layer.

In a second example of the invention, the multilayer cap has a Co-basedfirst layer which is on the order of about 100 angstroms thick orthicker, such as in the range of about 100 to about 300 angstroms, andcomprises a composition such as Co—B, Co—B—P, Co—W—B—P, or Co—B—Wdeposited employing an alkylamine borane such as DMAB, DEAB, ormorpholine borane as a reducing agent. A second Co-based layer also ofsimilar thickness greater than 100 angstroms is then deposited by analternative deposition process, such as by electroless Co depositionemploying a hypophosphite reducing agent or mixture of reducing agents.

In a third example of the invention, the multilayer cap has a firstlayer which is on the order of about 100 angstroms thick or thicker,such as in the range of about 100 to about 300 angstroms, and comprisesa composition such as Co—B, Co—B—P, Co—W—B—P, or Co—B—W deposited byself-initiated electroless deposition employing an alkylamine boranesuch as DMAB, DEAB, or morhpholine borane as a reducing agent. A secondNi-based layer (e.g., Ni—Co) or other non-Co-based layer also of greaterthan 100 angstroms is then deposited by an alternative depositionprocess, such as by conventional electroless deposition.

In a fourth example of the invention, the multilayer cap has a firstlayer which is a Co seed layer. As a seed layer, it is significantlydiscontinuous and uncoalesced. In the areas where there is coverage, itis especially thin (e.g., 10 angstroms or less). This seed layer may beapplied by conventional seeding techniques such as PVD or CVD of acatalytic metal or Co or Ni, or by self-initiated borane-chemistryelectroless deposition employing an alkylamine borane reducing agentsuch as DMAB, DEAB, or morpholine borane as reducing agent. A secondlayer is on the order of about 100 angstroms thick or thicker, such asin the range of about 100 to about 300 angstroms, and comprises Co-basedcompositions such as Co—W—P or Co—W—B, and is deposited by electrolessdeposition. A third layer is a Co-based, Ni-based, or other non-Co-basedlayer also greater than about 100 angstroms deposited by the same or analternative deposition process.

In a fifth example of the invention there are two or more adjacentlayers deposited using a) the same electroless plating bath underdifferent conditions, or b) different electroless baths under differentconditions, or c) different electroless plating baths under the sameconditions. Each layer is therefore compositionally modulated byadjustment of one or more deposition parameters. For example, each oftwo adjacent layers may be deposited using the same bath and identicalconditions except that the pH and/or temperature during deposition ofthe second adjacent layer is different from the pH and/or temperatureduring deposition of the first layer. Other conditions which can bemodulated to affect composition include tool-related effects, such asbath agitation, substrate rotation and rotation speed, solution flowrate, and the like. Or, the pH, temperature, and all the otherparameters are the same for each layer, but the compositions aredistinct in that the first layer comprises Co—B and a second layercomprises Co—B—W.

These five more specific, nonexclusive examples from among the severalembodiments within the invention can therefore be summarized as follows:

-   -   Ex I.        -   Layer 1: Co-based layer of low thickness formed by            electroless deposition using an alkylamine borane reducing            agent        -   Layer 2: Co-based layer >100 angstroms formed by electroless            deposition process distinct from the process of layer 1    -   Ex. II        -   Layer 1: Co-based layer >100 angstroms formed by electroless            deposition using alkylamine borane reducing agent        -   Layer 2: Co-based layer >100 angstroms formed by electroless            deposition process distinct from the process of layer 1    -   Ex. III        -   Layer 1: Co-based layer >100 angstroms formed by electroless            deposition using alkylamine borane reducing agent        -   Layer 2: Ni-based layer >100 angstroms    -   Ex. IV        -   Layer 1: Co-based seed layer        -   Layer 2: Co-based layer >100 angstroms formed by electroless            deposition        -   Layer 3: Co- or Ni-based electroless layer >100 angstroms    -   Ex. V        -   Layer 1: Co-based layer >100 angstroms formed by electroless            deposition        -   Layer 2: Co-based layer >100 angstroms formed by electroless            deposition

Alternatives within the foregoing embodiments are to employ two, three,or more distinct deposition operations such that “multi” of themultilayer cap refers to two, three, or more layers. From a strictlycost and process engineering perspective, it is most typical to employjust two layers, because less operations are involved. However, theremay be situations where three, four, or more layers are preferred. Forexample, three or four layers may be preferred in certain instances whenthe cap serves several functions. Other alternatives are to replace oneof the relatively thicker (100+ angstrom) layers with a layer ofmoderate thickness, such as between about 60 and about 100 angstroms.

One function performed by the cap of the invention, as noted above, isto serve as a barrier to electromigration of Cu. Ternary Co-based alloyshave been discovered to be especially advantageous for this purpose. Formany of the preferred embodiments of the invention at least one of thelayers employs one of the following alloys:

-   -   Co—W—P    -   Co—W—B    -   Co—W—B—P    -   Co—B—P    -   Co—B    -   Co—Mo—B    -   Co—W—Mo—B    -   Co—W—Mo—B—P    -   Co—Mo—P

The components of the ternary and more complex alloys for use with theinvention include a barrier element, Co, which is selected insignificant part because it is immiscible with Cu, and therefore doesnot tend to alloy with Cu during assembly or over time during service. Asecond component is the refractory element such as W, Mo, or Re whichfunctions to increase thermal stability, corrosion resistance, anddiffusion resistance. A third component is P or B present as aconsequence of decomposition of the reducing agent. An effect of theseelements is to reduce grain size, which can render the microstructuremore impervious to Cu electromigration. COWB with high W content has aphase which appears to approach overall amorphousness. Without beingbound to a particular theory, it is believed that the presence ofrefractory metal together with B and P improves the barrier propertiesby filling in the grain boundaries of the crystalline structure of thecapping film.

One or more of the layers of the cap may comprise a quaternary alloy.

Another function performed by the cap in many of the applications of theinvention, as noted above, is to provide an etch stop layer to inhibitdamage to interconnects during subsequent etching operations.

Another function which may be performed by the cap is resistance tochemical erosion.

In performing the process of the invention to deposit the cap of theinvention as described above, process steps are carried out includingpreparation of the substrate, self-initiated electroless deposition,surface activation, complexation rinsing, and electroless deposition, asdescribed in more detail below.

In preparing the substrate for capping, one or more acids is employedfor mildly etching interlayer dielectric (ILD), and one or more acids isemployed for dissolving Cu oxide. Alternatively, alkaline solutions maybe preferred for etching certain substrates.

A first pretreatment operation involves exposing the substrate to anacid selected from among HF, NH₄F, H₂SO₄ for mildly etching thedielectric to remove Cu embedded in the dielectric by CMP. For example,one known HF acid etch for this purpose is a 500:1 buffered hydroflouric(BHF) acid etch.

After the first pretreatment operation is completed, the substrate isrinsed by, e.g., DI water.

A second pretreatment employs an organic or inorganic acid or basiccleaner for removing oxide from the metal interconnect feature. Thiscleaner preferably removes all the oxide, for example copper oxide,without removing substantial amounts of the metallization in theinterconnects. Unless removed, the oxide can interfere with adhesion ofthe cap and can detract from electrical conductivity. Cleaners of thistype are known and typically contain an etching agent such as a weaksolution of an acid with less than 10 wt % in water of a strong mineralacid such as HF, HNO₃, or H₂SO₄ or a weak organic or carboxylic acidsuch as citric or malonic acid. Such cleaners also include a surfactantto help wet the surface, such as Rhodafac RE620 (Rhone-Poulenc). Analternative acid cleaner contains citric acid and boric acid at pH 6.5.

Typical basic cleaners contain TMAH with addition of hydroxlyamine, MEA,TEA, EDA (ethylenediamine), or DTA (diethylenetriamine) at pH range of 9to 12.

As a first layer on the metallization, as noted above, certainembodiments of the invention involve deposition of a first Co-basedlayer by electroless deposition employing borane chemistry. Thisexposure may comprise dip, flood immersion, spray, or other manner ofexposing the substrate to a deposition bath, with the provision that themanner of exposure adequately achieve the objectives of depositing aCo-based cap of the desired thickness and integrity.

For self-initiation of the electroless deposition, the invention employsa borane-based reducing agent such as an alkylamine borane reducingagent, for example DMAB, DEAB, morpholine borane, mixtures thereof, ormixtures thereof with hypophosphite. Oxidation/reduction reactionsinvolving the borane-based reducing agents and Co ions are catalyzed byCu. In particular, at certain plating conditions, e.g. pH & temperature,the reducing agents are oxidized in the presence of Cu, thereby reducingionic Co to Co metal which deposits on the Cu. One currently preferredreducing agent system employs about 1 g/L DMAB reducing agent in amixture with about 10 g/L hypophosphite reducing agent. The process issubstantially self-aligning in that the Co is deposited essentially onlyon the Cu interconnect, such that the process is maskless because thereis no need to mask areas other than the interconnect. Moreover, there isno need to subsequently remove substantial amounts of stray Codeposition from the dielectric.

The self-initiating electroless Co deposition bath comprises a source ofCo ions which are introduced into the solution as an inorganic Co saltsuch as chloride and/or sulfate or other suitable inorganic salts, or aninorganic complex such as pyrophosphate, or a Co complex with an organiccarboxylic acid such as Co acetate, citrate, lactate, succinate,propionate, hydroxyacetate, EDTA or others in the range of between about1 and about 20 g/L Co²⁺.

The bath may also contain an alkali-free source of refractory metalions. The level of refractory metal ions in the deposition bath isbetween 0 and up to on the order of about 50 g/L of refractory metalsalt in the solution. In one embodiment, the refractory metal isselected from among W, Re, Mo, and mixtures thereof.

The bath further contains one or more complexing agents and bufferingagents. The complexing agents used in the bath are selected from amongcitric acid, malic acid, glycine, propionic, succinic, lactic acids,DEA, TEA, and ammonium salts such as ammonium chloride, ammoniumsulphate, ammonium hydroxide, pyrophosphate, and mixtures thereof. Thebuffering agents are selected from among ammonium, borate, phosphate,acetate, and mixtures thereof. For pH adjustment, ammonium, TMAH, ormixtures thereof are typical for alkaline pH adjustment. Sulfuric,hydrochloric, and citric acids are used for acidic pH adjustment, withthe acid selection made to correlate to the anion of the Co source.

Within the above guidelines, one electroless bath for self-initiateddeposition contains the following constituents, by weight:

-   -   CoCl₂6H₂O25 g/L    -   Citric acid50 g/L    -   Tungstic acid4 g/L    -   DMAB1 g/L    -   TMAH ca. 30 g/L of a 25% TMAH solution for pH adjustment    -   Water Balance    -   pH 9.5

As an alternative first layer on the metallization, as noted above,certain embodiments of the invention employ an electroless Co-baseddeposition process which does not employ a reducing agent which rendersCu catalytic to Co deposition. For such processes a surface activationoperation is employed to facilitate subsequent electroless deposition. Acurrently preferred surface activation process utilizes a Pd immersionreaction. Other known hydrogenation/dehydrogenation catalysts fororganic synthesis are suitable and include Rh, Ru, Pt, Ir, and Os.Alternatively, the surface may be prepared for electroless deposition byseeding as with, for example, Co seeding deposited by PVD, CVD, or othertechnique as is known in the art.

Palladium chloride (PdCl₂) is a readily available Pd source for thepreferred surface activation process. The trend with the presentinvention is to bias the pH toward milder conditions, such as bybuffering the pH to 2 and higher by use of a phosphate or other knownbuffering agent. It has been discovered that by buffering the pH, excessetching of the Cu interconnects on the substrate is minimized. Inbuffering, a preferred source of borate is boric acid which isneutralized to tetramethyl ammonium borate in the working solution.

In the surface activation solution a ligand is required to keep the Pdions in solution. While maintaining Pd in solution, one challenge is notto form too highly complexed of a Pd complex, because the ultimate goalis to have the Pd release and deposit onto the Cu in reasonabledeposition times, such as about 30 seconds or less. Bromide is onepreferred ligand in the activation process of the invention. Bromide maybe provided by HBr. It has been discovered that bromide between about 50ppm and about several grams/L allows use of lower PdCl₂ (e.g., 10 ppm)concentration because bromide-Pd complexes are not too strong and nottoo weak. Bromide-Pd complexes are therefore not so strong that they donot readily release for deposition onto the substrate, yet they are notso weak that they release too readily such that selectivity is lost.With bromide-Pd complexes, therefore, selectivity is enhanced, whichprovides an added benefit that the amount of Pd in the solution can belower than if other complexes are employed. Such bromide levels achieveacceptable Pd deposition initiation rates of less than about 30 seconds.In particular, at 10 ppm Pd, 120 ppm bromide, and a pH of 4, acceptableinitiation rates are achieved with no appreciable Cu etch.

Sources of Pd²⁺ other than PdCl₂ include PdSO₄₁ PdBr₂, Pd(NO₃)₂,palladium acetate, and palladium propionate. The complexing ligand forstabilizing the Pd²⁺ is citrate, acetic acid, or MSA. An acid or mixtureof acids correlating to the anion of the Pd compound such as theexemplary inorganic acids hydrochloric, hydrobromic, sulfuric, andphosphoric is used for pH adjustment. Less aggressive acids arepreferred in many instances to alleviate problems of microtrenching atthe interface between Cu and Ta or other barrier on the dielectric. Suchacids are optionally organic acids such as organic aliphatic mono- &di-carboxylic acids (glycolic, succinic, oxalic, lactic, trifluoroacetic(halogen-substituted)), aromatic mono- & di-carboxylic acids (benzoic,phtalic), aromatic sulfonic or sulfinic (benzenesulfonic,toluenesulfonic, cumenesulfonic, xylenesulfonic, phenolsulfonic,cresolsulfonic, naphtalenesulfonic, and analogous-sulfinics), aromaticphosphonic or phosphinic, or inorganic Acids with reducing properties(hypophosphorous, sulfurous). Palladium sources are typically, but notnecessarily, selected to have an anion correlating to such acids notnecessarily.

A second challenge in formulating activation chemistry is selectivity ofdeposition for the metal over the dielectric, to avoid formingactivation sites on the dielectric surface. This challenge is addressedby combination of pre-cleaning, Pd activation chemistry, and postactivation cleaning/complexing rinse as described herein.

The process is substantially self-aligning in that the activator isdeposited essentially only on the Cu interconnect, such that the processis maskless because there is no need to mask areas other than theinterconnect.

A surfactant such as Calsoft LAS99 can optionally be used to improvewetting.

After activation, the substrate is exposed to a complexing rinse, whichfunctions to remove residual ionic activator left behind by theactivator composition, and thereby minimize subsequent deposition onunintended surfaces.

The invention in several aspects also involves electroless depositionwhich employs reducing agents other than the borane-based reducingagents described above. In particular, such deposition is one of theoptions used when forming a) a first layer, provided the Cu substrate iseither activated or seeded as described above, b) a second layer on topof a Co-based first layer, or c) a third or subsequent layer on top of asecond or subsequent layer. In aspect (a), the invention employs anelectroless metal deposition layer deposited with the assistance of areducing agent which i) renders a metal seed on the Cu catalytic todeposition, or ii) assists with Pd-catalyzed deposition of an activatedCu substrate. In aspect (b), the invention employs an electroless Coand/or Ni and/or other metal deposition layer deposited with theassistance of a reducing agent which renders the previously appliedCo-based layer catalytic to deposition of the next layer. To achievethis, for example, an electroless bath is employed comprising a sourceof Co ions and a reducer. The Co ions are introduced into the solutionas an inorganic Co salt such as chloride and/or sulfate or a Co complexwith an organic carboxylic acid such as Co acetate, citrate, lactate,succinate, propionate, or hydroxyacetate in the range of between about 2and about 50 g/L Co.

One or more reducing agents such as a hypophosphite reducing agent(e.g., ammonium hypophosphite) is employed in a concentration roughlybetween about 2 and about 30 g/L. The reducing agent is alkali-free; andparticularly Na-free. Other reducing agents are described in Mallory andHajdu (Eds.), Electroless Plating, Fundamentals and Applications,American Electroplaters and Surface Finishers Society (1990), as well asnumerous electroless bath compositions and reaction chemistries for Niand Co electroless plating. In one embodiment a hypophosphite or otherreducing agent is supplemented with a borane-based reducing agent.

The bath further may also contain an alkali-free source of refractorymetal ions. The level of refractory metal ions in the deposition bath ina concentration between 0 and up to on the order of about 50 g/Lrefractory metal salt. In one embodiment, the refractory metal isselected from among W, Re, Mo, and mixtures thereof.

In one especially preferred embodiment, W ions are provided by analkali-free, tungstate-based source of W ions. Preferred among suchsources of W ions are tetramethylammonium tungstate, phosphotungstate,silicotungstate, and mixtures thereof. For example, one preferreddeposition bath contains between about 2 and about 10 g/L tungstic acidneutralized by TMAH. Other sources of refractory metal include ammoniummolybdate.

A stabilizer for the Co is incorporated into one preferred embodiment ofthe electroless Co deposition bath. Though the Co complexes are stablein solution in the absence of a reducing agent, the use of a stabilizercan help prevent spontaneous decomposition of the bath components.Exemplary stabilizers include, for example, Pb, Bi, Sn, Sb, IO₃, MoO₃,AsO₃ azoles such as imidazole and derivatives. The stabilizer level ison the order of between about 0.1 and about 100 ppm. For example, forPb+² from about 5 to about 20 ppm has been shown to be effective. ForMoO₄-2, about 10 to about 100 ppm has been shown to be effective. Onepreferred stabilizer is maleic acid, which is a stabilizer within thefour classes discussed in Mallory and Hajdu (Eds.), Electroless Plating,Fundamentals and Applications, American Electroplaters and SurfaceFinishers Society (1990) (p. 34-44).

A grain refiner for the deposited Co is optionally incorporated into thebath. This grain refiner can also be characterized as a stabilizer, butthis grain refiner is distinguished from the stabilizers described abovein that those stabilizers do not primarily perform a grain refiningfunction. Exemplary grain refiners include, for example, Cd, Cu, Al,saccharine, 2-butyne-1,4,-diol and its alkoxylates, 3-hexyne-2,5,-diol,propargyl alcohol and its alkoxylates and sulfonates, alyl sulfonate,betaine and its sulfobetaine derivatives.

The bath typically contains a pH buffer to stabilize the pH in thedesired range. In one embodiment, the desired pH range is between about8.0 and about 10.0. If the pH is not stabilized, unintentional,undesirable, and unanticipated changes in deposition rate and depositchemistry can occur. Exemplary buffers include, for example, borates,tetra- and pentaborates, phosphates, acetates, glycolates, lactates,ammonia, and pyrophosphate. The pH buffer level is on the order ofbetween about 10 and about 50 g/L.

A complexing agent for the Co is included in the bath to help keep theCo in solution. The bath is typically operated at a pH of between about8 and about 10, at which level the Co can have a tendency to formhydroxides and precipitate out of solution. Exemplary complexing agentsinclude citrate, glycolic acid, lactic acid, malic acid, succinic acid,pyophosphate, ammonium, DEA, TEA, and EDTA. The complexing agentconcentration is selected such that the molar ratio between thecomplexing agent and Co is between about 2:1 and about 4:1, generally.Depending on the complexing agent molecular weight, the level ofcomplexing agent may be on the order of between about 20 g/L and about120 g/L.

One or more surfactants is optionally included in the bath to help wetthe substrate surface. The surfactant also can serve as a milddeposition inhibitor which can suppress three-dimensional growth to anextent, thereby improving morphology and topography of the film. It canalso help refine the grain size, which yields a more uniform coatingwhich has grain boundaries which are less porous to migration of Cu.Exemplary anionic surfactants include alkyl phosphonates, alkyl etherphosphates, alkyl sulfates, alkyl ether sulfates, alkyl sulfonates,alkyl ether sulfonates, carboxylic acid ethers, carboxylic acid esters,alkyl aryl sulfonates, and sulfosuccinates. Exemplary non-ionicsurfactants include alkoxylated alcohols, ethoxy/propoxy (EO/PO) blockcopolymers, alkoxylated fatty acids, glycol and glycerol esters, withpolyethylene glycols, and polypropylene glycol/polyethylene glycolcurrently preferred. The level of surfactant is on the order of betweenabout 0.01 and about 5 g/L.

Within the above guidelines, one electroless bath for deposition asdisclosed in Shacham-Diamand, Journal of Electrochemical Society“Electroless Deposition of Thin-Film Co—W—P Layers” (2001) contains thefollowing constituents: 23 g/L CoSO₄—H₂O 21 g/L NaH₂PO₂ 130 g/L  Nacitrate 10 g/L Na₂WO₄—H₂O 0.05 g/L   Rhodafac RE-610 Balance water pHabout 9.0

Another suitable bath disclosed therein contains the following: 23 g/LCoSO₄—H₂O 21 g/L NaH₂PO₂ 130 g/L  Na citrate 10 g/L H₃P(W₃O₁₀)₄ 0.05g/L   Rhodafac RE-610 Balance water pH about 9.0

A further suitable bath is disclosed in Pat. No. 5,695,810: 10 g/L(NH₄)₂WO₄ 30 g/L CoCl₂—6H₂O 80 g/L Na₃C₆H₄O₇—2H₂O 20 g/L Na₂H₂PO₂ 0.05g/L   Rhodafac RE610

This electroless process, like the above-described process employing aborane-based reducing agent, is substantially self-aligning in that theelectroless metal is deposited essentially only onto the metalinterconnect, or onto a previously deposited metal cap layer in thesituation of a second or subsequent layer in multilayer deposition. Theprocess therefore does not require lithography and its several steps ofpatterning, stripping etc. Moreover, there is no need to subsequentlyremove substantial amounts of unintended cap metal deposition from thedielectric.

While the emphasis in the above-described preferred embodiments is onelectroless deposition processes, one or more of the layers of themultilayer cap may alternatively be applied by a metal immersion processas is known in the art. In immersion plating, also known as displacementplating, a metal on a surface is displaced by a metal ion in animmersion reaction. The driving force is a lower oxidation potential ofthe metal ion in the solution. Immersion plating parameters aredisclosed in Hirsch et al., “Immersion Plating,” Metal FinishingGuidebook, 1989, pp. 402-406. In immersion plating, the base metal onthe surface functions as the reducing agent. Immersion plating thereforediffers from electroless plating in that a further reducing agent is notrequired. Another distinction is that, in theory, in immersion platingthe plating stops once the surface is covered with the metal beingdeposited. In this regard it is self-limiting. In contrast, electrolessplating is auto-catalytic and, in theory, it continues as long as thereis ample reducing agent in the solution.

In performing the invention to produce a multilayer cap and integratedcircuit substrate of the invention, the substrate is a dielectricmaterial in which one or more, typically many, interconnect featureshave been formed. In depositing the multilayer cap on a metal-filledinterconnect feature, the cap is built up on top of the interconnectfeature and, dimensionally, forms an extension comprising layers 18 and20 (FIG. 1) on the top of the interconnect feature. Accordingly, theinterconnect/cap combination is dimensionally larger than theinterconnect prior to capping.

The exposed surface of an interconnect feature prior to capping istypically roughly co-planar with a flat top surface of the dielectricmaterial. This co-planarity is achieved by CMP to remove metal—metaltypically applied over the entire dielectric and into interconnectfeatures by a previous electrodeposition operation—from the top surfaceof the dielectric. The only metal left on the substrate after the CMPoperation is the exposed metal in the interconnect features, plusperhaps some trace metal embedded into the dielectric by the CMP, thoughthis trace metal is typically removed by an above-described cleaningoperation. In instances where an interconnect feature prior to cappingis co-planar with a flat top surface of the dielectric material, themultilayer cap applied by this invention extends above the plane definedby the flat top surface of the dielectric material with which theexposed surface of the interconnect feature is co-planar. Depositing thecap in these instances therefore involves building cap material on topof the metal interconnect such that the cap extends above the plane ofthe dielectric.

The multilayer cap, which dimensionally constitutes an extension on topof the wiring, is permanent in that it is not removed in subsequentprocessing. For example, in forming multilayer devices, an etch stoplayer is typically applied over the entire top surface of the dielectricand any cap extensions covering interconnects. In the present invention,this is performed without any intermittent chemical, mechanical, orother operation to remove the cap extensions. Most typically, the etchstop layer is applied directly over the top surface of the dielectricand directly over any cap extensions covering interconnects. This isalso accomplished without an intermittent annealing operation becausethe Co-based cap serves its protective capping function without such ananneal.

Multiple layers of dielectric and circuitry are then built byconventional means. The eventual multilayer integrated circuit devicemay be incorporated into an electronic package by way of an externalelectrical connector, such as a wire bond structure. The devicecomprises a series of dielectric layers between which are metalinterconnects with caps thereon. The multilayer cap of the invention isa barrier on top of Cu interconnects embedded in dielectric.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

When introducing elements of the present invention or the preferredembodiment(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. Forexample, that the foregoing description and following claims refer to“an” interconnect means that there are one or more such interconnects.The terms “comprising”, “including” and “having” are intended to beinclusive and mean that there may be additional elements other than thelisted elements.

As various changes could be made in the above without departing from thescope of the invention, it is intended that all matter contained in theabove description and shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

1. A method for forming a multilayer metal cap over a metal-filledinterconnect feature in a dielectric layer for incorporation into amultilayer integrated circuit device comprising: depositing a firstmetal cap layer over the metal-filled interconnect feature in a firstdeposition process which constitutes electroless metal deposition froman electroless bath comprising a source of metal ions and a reducingagent; and depositing a second metal cap layer over the first metal caplayer in a second deposition process distinct from the first depositionprocess to thereby form the multilayer metal cap as a permanentcomponent distinct from the metal-filled interconnect feature.
 2. Themethod of claim 1 wherein: the second deposition process is anelectroless deposition process; and the first deposition process isdistinct from the second deposition process in that the first depositionprocess employs a first electroless solution which has a first solutioncomposition distinct from, and prepared separately from, a secondelectroless solution having a second solution composition employed inthe second deposition process.
 3. The method of claim 1 wherein thefirst deposition process employs a first electroless solution comprisinga source of Co ions and a borane-based reducing agent.
 4. The method ofclaim 1 wherein the first deposition process is self-aligning in thatthe first metal cap layer is selectively deposited onto the metal-filledinterconnect and the second deposition process is self-aligning in thatthe second metal cap layer is selectively deposited onto the first metalcap layer.
 5. The method of claim 1 wherein the metal-filledinterconnect feature is a Cu-filled interconnect feature, and the firstmetal cap layer comprises a metal or metal alloy which is substantiallyimmiscible with Cu.
 6. The method of claim 1 wherein: the firstdeposition process employs a first electroless solution comprising asource of Co ions and a borane-based reducing agent; the seconddeposition process is electroless and employs a second electrolesssolution which comprises a source of Co ions and a reducing agent, andwhich is distinct from, and prepared separately from, the firstelectroless solution.
 7. The method of claim 1 wherein: the firstdeposition process employs a first electroless solution comprising asource of Co ions and a borane-based reducing agent; the seconddeposition process is electroless and employs a second electrolesssolution which comprises a source of Ni ions and a reducing agent. 8.The method of claim 1 wherein: the first deposition process employs afirst electroless solution comprising a source of Co ions and aborane-based reducing agent; the second deposition process iselectroless and employs a second electroless solution which comprises asource of Ni ions and a reducing agent.
 9. The method of claim 1 whereinthe second deposition process is an electroless deposition processdistinct from the first deposition process with respect to a parameterselected from among pH, deposition temperature, liquid flow rate, bathagitation, substrate rotation, and combinations thereof.
 10. The methodof claim 1 wherein the first metal cap layer is substantially continuousand has a thickness between about 5 and about 50 angstroms.
 11. Themethod of claim 1 wherein: the first metal cap layer is substantiallycontinuous and has a thickness between about 5 and about 50 angstroms;and the second metal cap layer has a thickness greater than about 100angstroms.
 12. The method of claim 1 wherein the first metal cap layeris discontinuous and substantially uncoalesced.
 13. The method of claim1 comprising discontinuing deposition between the first depositionprocess and the second deposition process.
 14. The method of claim 1wherein the first metal cap layer and second metal cap layer compriseCo-based alloys.
 15. The method of claim 1 wherein the first metal caplayer comprises a Co-based alloy and the second metal cap layercomprises a Ni-based alloy.
 16. The method of claim 1 further comprisingdepositing a third metal cap layer over the second metal cap layer in athird deposition process distinct from the second deposition process.17. The method of claim 1 comprising depositing a discontinuous,substantially uncoalesced Co seed onto the metal-filled interconnectfeature prior to depositing the first metal cap layer.
 18. The method ofclaim 1 wherein: the metal-filled interconnect feature is a Cu-filledinterconnect feature; the second deposition process is an electrolessdeposition process; and the first deposition process is distinct fromthe second deposition process in that the first deposition processemploys a first electroless solution which has a first solutioncomposition distinct from, and prepared separately from, a secondelectroless solution having a second solution composition employed inthe second deposition process.
 19. The method of claim 1 wherein: themetal-filled interconnect feature is a Cu-filled interconnect feature;the second deposition process is an electroless deposition process whichis self-aligning in that the second metal cap layer is selectivelydeposited on the first metal cap layer; the first deposition process isself-aligning in that the second metal cap layer is selectivelydeposited on the first metal cap layer; and the first deposition processis distinct from the second deposition process in that the firstdeposition process employs a first electroless solution which has afirst solution composition distinct from, and prepared separately from,a second electroless solution having a second solution compositionemployed in the second deposition process.
 20. The method of claim 1wherein: the metal-filled interconnect feature is a Cu-filledinterconnect feature; the second deposition process is an electrolessdeposition process; the first deposition process is distinct from thesecond deposition process in that the first deposition process employs afirst electroless solution which has a first solution compositiondistinct from, and prepared separately from, a second electrolesssolution having a second solution composition employed in the seconddeposition process; the second metal cap layer has a thickness greaterthan about 100 angstroms; and the first metal cap layer comprises aCo-based alloy and has a thickness greater than about 100 angstroms. 21.The method of claim 1 wherein: the metal-filled interconnect feature isa Cu-filled interconnect feature; the second deposition process is anelectroless deposition process; the first deposition process is distinctfrom the second deposition process in that the first deposition processemploys a first electroless solution which has a first solutioncomposition distinct from, and prepared separately from, a secondelectroless solution having a second solution composition employed inthe second deposition process; the second metal cap layer has athickness greater than about 100 angstroms; and the first metal caplayer comprises a Co-based alloy, is discontinuous and substantiallyuncoalesced, and has a thickness between about 5 and about 50 angstroms.22. The method of claim 1 wherein: the metal-filled interconnect featureis a Cu-filled interconnect feature; the second deposition process is anelectroless deposition process; the first deposition process is distinctfrom the second deposition process in that the first deposition processemploys a first electroless solution which has a first solutioncomposition distinct from, and prepared separately from, a secondelectroless solution having a second solution composition employed inthe second deposition process; the second metal cap layer comprises aNi-based alloy and has a thickness greater than about 100 angstroms; andthe first metal cap layer comprises a Co-based alloy and has a thicknessgreater than about 100 angstroms.
 23. The method of claim 1 wherein: themetal-filled interconnect feature is a Cu-filled interconnect feature;the second deposition process is an electroless deposition process; thefirst deposition process is distinct from the second deposition processin that the first deposition process employs a first electrolesssolution which has a first solution composition distinct from, andprepared separately from, a second electroless solution having a secondsolution composition employed in the second deposition process; thesecond metal cap layer comprises a Ni-based alloy and has a thicknessgreater than about 100 angstroms; and the first metal cap layercomprises a Co-based alloy, is discontinuous and substantiallyuncoalesced, and has a thickness between about 5 and about 50 angstroms.24. The method of claim 1 wherein: the metal-filled interconnect featureis a Cu-filled interconnect feature; the second deposition process is anelectroless deposition process; the first deposition process is distinctfrom the second deposition process in that the first deposition processemploys a first electroless solution which has a first solutioncomposition distinct from, and prepared separately from, a secondelectroless solution having a second solution composition employed inthe second deposition process; the first electroless solutioncomposition comprises a borane-based compound as its primary reducingagent; the first second electroless solution composition comprises anon-borane-based compound as its primary reducing agent; the secondmetal cap layer has a thickness greater than about 100 angstroms; andthe first metal cap layer comprises a Co-based alloy and has a thicknessgreater than about 100 angstroms.
 25. The method of claim 1 wherein: themetal-filled interconnect feature is a Cu-filled interconnect feature;the second deposition process is an electroless deposition process; thefirst deposition process is distinct from the second deposition processin that the first deposition process employs a first electrolesssolution which has a first solution composition distinct from, andprepared separately from, a second electroless solution having a secondsolution composition employed in the second deposition process; thefirst electroless solution composition comprises a borane-based compoundas its primary reducing agent; the first second electroless solutioncomposition comprises a non-borane-based compound as its primaryreducing agent; the second metal cap layer has a thickness greater thanabout 100 angstroms; and the first metal cap layer comprises a Co-basedalloy and has a thickness between about 5 and about 50 angstroms. 26.The method of claim 1 wherein: the metal-filled interconnect feature isa Cu-filled interconnect feature; the second deposition process is anelectroless deposition process; the first deposition process is distinctfrom the second deposition process in that the first deposition processemploys a first electroless solution which has a first solutioncomposition distinct from, and prepared separately from, a secondelectroless solution having a second solution composition employed inthe second deposition process; the first electroless solutioncomposition comprises a borane-based compound as its primary reducingagent; the second electroless solution composition comprises ahypophosphite compound as its primary reducing agent; the second metalcap layer has a thickness greater than about 100 angstroms; and thefirst metal cap layer comprises a Co-based alloy and has a thicknessgreater than about 100 angstroms.
 27. The method of claim 1 wherein: themetal-filled interconnect feature is a Cu-filled interconnect feature;the second deposition process is an electroless deposition process; thefirst deposition process is distinct from the second deposition processin that the first deposition process employs a first electrolesssolution which has a first solution composition distinct from, andprepared separately from, a second electroless solution having a secondsolution composition employed in the second deposition process; thefirst electroless solution composition comprises a borane-based compoundas a primary reducing agent; the second electroless solution compositioncomprises a hypophosphite compound as a primary reducing agent; thesecond metal cap layer has a thickness greater than about 100 angstroms;and the first metal cap layer comprises a Co-based alloy and has athickness between about 5 and about 50 angstroms.
 28. A method forforming a multilayer metal cap over a metal-filled interconnect featurein a dielectric layer for incorporation into a multilayer integratedcircuit device comprising: depositing a first metal cap layer over themetal-filled interconnect feature in a first deposition process whichconstitutes noble metal immersion deposition from a solution comprisinga source of noble metal ions; depositing a second metal cap layer overthe first metal cap layer in a second deposition process distinct fromthe first deposition process; and depositing a third metal cap layerover the second metal cap layer in a third deposition process distinctfrom the second deposition process.
 29. The method of claim 28 wherein:depositing the second metal cap layer over the first metal cap layer inthe second deposition process constitutes electroless metal depositionfrom an electroless bath comprising a source of metal ions and areducing agent.
 30. The method of claim 28 wherein: depositing thesecond metal cap layer over the first metal cap layer in the seconddeposition process constitutes electroless metal deposition from anelectroless bath comprising a source of metal ions and a reducing agent;and depositing the third metal cap layer over the second metal cap layerin the third deposition process constitutes electroless metal depositionfrom an electroless bath comprising a source of metal ions and areducing agent.
 31. The process of claim 28 wherein: depositing thefirst metal cap layer comprises depositing a Pd layer which isdiscontinuous and substantially uncoalesced, and functions as a seedlayer for the second deposition process.
 32. The process of claim 28wherein: depositing the first metal cap layer comprises depositing a Pdlayer which is discontinuous and substantially uncoalesced, andfunctions as a seed layer for the second deposition process; anddepositing the second metal cap layer over the first metal cap layer inthe second deposition process constitutes electroless metal depositionfrom an electroless bath comprising a source of metal ions and areducing agent.
 33. The process of claim 28 wherein: depositing thefirst metal cap layer comprises depositing a Pd layer which isdiscontinuous and substantially uncoalesced, and functions as a seedlayer for the second deposition process; depositing the second metal caplayer over the first metal cap layer in the second deposition processconstitutes electroless metal deposition from an electroless bathcomprising a source of metal ions and a reducing agent; and depositingthe third metal cap layer over the second metal cap layer in the thirddeposition process constitutes electroless metal deposition from anelectroless bath comprising a source of metal ions and a reducing agent.34. The process of claim 28 wherein depositing the first metal cap layercomprises depositing a Pd layer which is substantially continuous andhas a thickness between about 5 and about 50 angstroms.
 35. The processof claim 28 wherein: depositing the first metal cap layer comprisesdepositing a Pd layer which is substantially continuous and has athickness between about 5 and about 50 angstroms; and depositing thesecond metal cap layer over the first metal cap layer in the seconddeposition process constitutes electroless metal deposition from anelectroless bath comprising a source of metal ions and a reducing agent.36. The process of claim 28 wherein: depositing the first metal caplayer comprises depositing a Pd layer which is substantially continuousand has a thickness between about 5 and about 50 angstroms; depositingthe second metal cap layer over the first metal cap layer in the seconddeposition process constitutes electroless metal deposition from anelectroless bath comprising a source of metal ions and a reducing agent;and depositing the third metal cap layer over the second metal cap layerin the third deposition process constitutes electroless metal depositionfrom an electroless bath comprising a source of metal ions and areducing agent.
 37. The method of claim 28 wherein: the third depositionprocess is an electroless deposition process; and the second depositionprocess is distinct from the third deposition process in that the seconddeposition process employs a second electroless solution which has asecond solution composition distinct from, and prepared separately from,a third electroless solution having a third solution compositionemployed in the third deposition process.
 38. The method of claim 28wherein the second deposition process employs an electroless solutioncomprising a source of Co ions and a reducing agent.
 39. The method ofclaim 28 wherein the second deposition process is self-aligning in thatthe second metal cap layer is selectively deposited onto the first metalcap layer and the third deposition process is self-aligning in that thethird metal cap layer is selectively deposited onto the second metal caplayer.
 40. The method of claim 28 wherein the metal-filled interconnectfeature is a Cu-filled interconnect feature, and the second metal caplayer comprises a metal or metal alloy which is substantially immisciblewith Cu.
 41. The method of claim 28 wherein: the second depositionprocess employs a second electroless solution comprising a source of Coions and a reducing agent; the third deposition process is electrolessand employs a third electroless solution which comprises a source of Coions and a reducing agent, and which is distinct from, and preparedseparately from, the second electroless solution.
 42. The method ofclaim 28 wherein: the second deposition process employs a secondelectroless solution comprising a source of Co ions and a reducingagent; the third deposition process is electroless and employs a thirdelectroless solution which comprises a source of Ni ions and a reducingagent.
 43. The method of claim 28 wherein the third deposition processis an electroless deposition process distinct from the second depositionprocess with respect to a parameter selected from between pH anddeposition temperature.
 44. The method of claim 28 wherein: the firstmetal cap layer is substantially continuous and has a thickness betweenabout 5 and about 50 angstroms; and the second metal cap layer has athickness greater than about 100 angstroms.
 45. The method of claim 28comprising discontinuing deposition between the first deposition processand the second deposition process, and between the second depositionprocess and the third deposition process.
 46. The method of claim 28wherein the second metal cap layer and third metal cap layer compriseCo-based alloys.
 47. The method of claim 28 wherein the second metal caplayer comprises a Co-based alloy and the third metal cap layer comprisesa Ni-based alloy.
 48. The method of claim 28 further comprisingdepositing a fourth metal cap layer over the third metal cap layer in afourth deposition process distinct from the third deposition process.49. The method of claim 28 wherein: the metal-filled interconnectfeature is a Cu-filled interconnect feature; the third depositionprocess is an electroless deposition process; and the second depositionprocess is distinct from the third deposition process in that the seconddeposition process employs a second electroless solution which has asecond solution composition distinct from, and prepared separately from,a third electroless solution having a third solution compositionemployed in the third deposition process.
 50. The method of claim 28wherein: the metal-filled interconnect feature is a Cu-filledinterconnect feature; the second deposition process is an electrolessdeposition process which is self-aligning in that the second metal caplayer is selectively deposited on the first metal cap layer; the thirddeposition process is self-aligning in that the third metal cap layer isselectively deposited on the second metal cap layer; and the seconddeposition process is distinct from the third deposition process in thatthe second deposition process employs a second electroless solutionwhich has a second solution composition distinct from, and preparedseparately from, a third electroless solution having a third solutioncomposition employed in the third deposition process.
 51. The method ofclaim 1 wherein: the metal-filled interconnect feature is a Cu-filledinterconnect feature; the third deposition process is an electrolessdeposition process; the second deposition process is distinct from thethird deposition process in that the second deposition process employs asecond electroless solution which has a second solution compositiondistinct from, and prepared separately from, a third electrolesssolution having a third solution composition employed in the thirddeposition process; the third metal cap layer has a thickness greaterthan about 100 angstroms; and the second metal cap layer comprises aCo-based alloy and has a thickness greater than about 100 angstroms. 52.The method of claim 28 wherein: the metal-filled interconnect feature isa Cu-filled interconnect feature; the second and third depositionprocesses are electroless deposition processes; the second depositionprocess is distinct from the third deposition process in that the seconddeposition process employs a second electroless solution which has asecond solution composition distinct from, and prepared separately from,a third electroless solution having a third solution compositionemployed in the third deposition process; the third metal cap layercomprises a Ni-based alloy and has a thickness greater than about 100angstroms; and the second metal cap layer comprises a Co-based alloy andhas a thickness greater than about 100 angstroms.
 53. A method forforming a multilayer metal cap over a metal-filled interconnect featurein a dielectric layer for incorporation into a multilayer integratedcircuit device comprising: depositing a catalytic first metal cap layerover the metal-filled interconnect feature in a first depositionprocess; and depositing a second metal cap layer over the first metalcap layer in a second deposition process distinct from the firstdeposition process, wherein the second deposition process comprisesautocatalytic deposition and deposition catalyzed by the first metal caplayer.
 54. The method of claim 53 wherein the first deposition processcomprises noble metal immersion deposition from a solution comprising asource of noble metal ions.
 55. The method of claim 53 wherein the firstdeposition process comprises Pd immersion deposition from a solutioncomprising a source of Pd ions, and the second deposition processcomprises electroless deposition of a Co-based layer employing ahypophosphite based reducing agent and a borane-based reducing agent.56. A multilayer cap over a metal-filled interconnect feature in adielectric layer for incorporation into a multilayer integrated circuitdevice comprising: a first metal cap layer over the metal-filledinterconnect feature; and a second metal cap layer over the first metalcap layer, wherein the first and second metal cap layers are componentsof the multilayer metal cap which constitutes a permanent componentdistinct from the metal interconnect feature.
 57. The multilayer cap ofclaim 56 wherein: the first metal cap layer comprises a metal or metalalloy which is substantially immiscible with the metal-filledinterconnect feature; and the first metal cap layer and second metal caplayer comprise metals or metal alloys which are substantially immisciblewith the dielectric layer.
 58. The multilayer cap of claim 56 whereinthe first metal cap layer comprises Co or a Co-based alloy.
 59. Themultilayer cap of claim 56 wherein: the first metal cap layer comprisesCo or a Co-based alloy; and the second metal cap layer comprises Co or aCo-based alloy.
 60. The multilayer cap of claim 56 wherein: the firstmetal cap layer comprises Co or a Co-based alloy; and the second metalcap layer comprises Ni or a Ni-based alloy.
 61. The multilayer cap ofclaim 56 wherein: the first metal cap layer has a thickness greater thanabout 100 angstroms; and the second metal cap layer has a thicknessgreater than about 100 angstroms.
 62. The multilayer cap of claim 56wherein: the first metal cap layer has a thickness between about 5 andabout 50 angstroms; and the second metal cap layer has a thicknessgreater than about
 100. 63. The multilayer cap of claim 56 wherein: thefirst metal cap layer has a thickness greater than about 100 angstromsand comprises Co or a Co-based alloy; and the second metal cap layer hasa thickness greater than about 100 angstroms and comprises Co or aCo-based alloy.
 64. The multilayer cap of claim 56 wherein: the firstmetal cap layer has a thickness greater than about 100 angstroms andcomprises Co or a Co-based alloy; and the second metal cap layer has athickness greater than about 100 angstroms and comprises Ni or aNi-based alloy.
 65. The multilayer cap of claim 56 wherein: the firstmetal cap layer has a thickness between about 5 and about 50 angstromsand comprises Co or a Co-based alloy; and the second metal cap layer hasa thickness greater than about 100 angstroms and comprises Co or aCo-based alloy.
 66. The multilayer cap of claim 56 wherein: the firstmetal cap layer has a thickness between about 5 and about 50 angstromsand comprises Co or a Co-based alloy; and the second metal cap layer hasa thickness greater than about 100 angstroms and comprises Ni or aNi-based alloy.
 67. The multilayer cap of claim 56 wherein there is adiscontinuous, substantially uncoalesced metal seed layer between thefirst metal cap layer and the metal-filled interconnect.